Electrophoretic material, electrophoresis display device, and electronic apparatus

ABSTRACT

In an electrophoretic material, first particles which are charged with a first polarity and second particles which are charged with a second polarity are dispersed in a solvent. A volume, which is obtained by dividing a volume of the solvent by a total number of particles, is called free volume, and a radius of a spherical space, which is occupied by sum of an average volume of the particles and the free volume, is called a free volume radius. A first particle average radius is greater than a second particle average radius. A difference between the free volume radius and the first particle average radius is greater than the second particle average radius.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation application of application Ser. No. 13/937,653,filed on Jul. 9, 2013, which claims priority to Japanese Application JP2012-157224. The entire disclosures of the prior applications are herebyincorporated by reference herein in their entireties.

BACKGROUND

1. Technical Field

The present invention relates to technical fields of an electrophoreticmaterial, an electrophoresis display device, and an electronicapparatus.

2. Related Art

In an electrophoresis display device, an image is formed on a displayarea by applying a voltage between a pixel electrode and a commonelectrode which are opposite to each other while interposing anelectrophoretic material, and causing charged electrophoretic particles,such as white particles and black particles, to spatially migrate. Ifthe electrophoretic particles once spatially migrate, diffusion issuppressed due to the cohesive force between the particles even when thevoltage is removed. Therefore, the electrophoresis display device has aproperty of maintaining the image even when power is not supplied, andthus it is applied to a low-power consumption digital book, or the like.In an electrophoretic material in the related art, white particles, suchas titanium oxide or the like, and red particles, such as dyed acrylicresin particles or the like, are used for electrophoretic particles asdisclosed in JP-A-2011-118417. Specifically, a titanium oxide of 8 gwhich has an average particle diameter of 0.25 micrometer (μm) and anacrylic resin particle of 8 g which has an average particle diameter of4 micrometer (μm) are dispersed in a dodecyl benzene of 80 mL, and areused as an electrophoretic material.

However, the electrophoretic material in the related art have problemsin that a response speed with regard to an applied voltage is slow andthat it is difficult to be driven at a low voltage. In other words, theelectrophoretic material in the related art has a problem of badelectrophoretic properties (low voltage driving and high-speedresponse).

SUMMARY

The invention can be realized in the following forms or applicationexamples.

Application Example 1

According to Application Example 1, there is provided an electrophoreticmaterial, in which the particles are dispersed in a solvent, volume,which is obtained by dividing a volume of the solvent by a total numberof particles, is called free volume, and a radius of a spherical space,which is occupied by sum of an average volume of the particles and thefree volume, is called a free volume radius. The particles include atleast first particles which are charged with a first polarity and secondparticles which are charged with a second polarity. An average radius ofthe first particles is called a first particle average radius, and anaverage radius of the second particles is called a second particleaverage radius. The first particle average radius is greater than thesecond particle average radius, a difference between the free volumeradius and the first particle average radius is greater than the secondparticle average radius, and an electric charge amount of the secondparticles is greater than an electric charge amount of the firstparticles.

In the electrophoretic material, since the electric charge amount ofsecond particles having a small particle size is great, theelectrophoretic mobility of the second particles is greater than theelectrophoretic mobility of the first particles, and thus it is easy toperform electrophoresis. On the other hand, since the second particleaverage radius is smaller than a distance which is obtained bysubtracting the first particle average radius from the free volumeradius, the second particles can easily pass through the gaps betweenthe first particles. In short, the second particles having the largeelectrophoretic mobility can easily pass through the gaps between thefirst particles. Therefore, the response speed of the electrophoreticmaterial is increased, and thus it is possible to drive theelectrophoretic material at a low voltage. In other words, it ispossible to provide the electrophoretic material having excellentelectrophoretic properties.

Application Example 2

According to the configuration according to Application Example 1, it ispreferable that surfaces of the second particles be coated with polymermolecules which include a functional group having the second polarity.

In the electrophoretic material, if the surfaces of the first particlesare coated with polymer molecules which include a functional grouphaving the non-polarity or the first polarity, it is possible to chargethe first particles with the first polarity, and it is possible tocharge the second particles with the second polarity. Further, since itis possible to independently control the electric charge amount of firstparticles and the electric charge amount of second particles, it ispossible to cause the electric charge amount of second particles to begreater than the electric charge amount of first particles.

Application Example 3

In the electrophoretic material according to Application Example 1, itis preferable that surfaces of the first particles and the secondparticles be coated with non-polarity polymer molecules, and a coatratio due to the non-polarity polymer molecules on the surfaces of thefirst particles be greater than a coat ratio due to the non-polaritypolymer molecules on the surfaces of the second particles.

If the particles are dispersed in the solvent, an electric chargeseparation phenomenon, which moderates work function difference betweenthe particles and the solvent, occurs, and thus the particles arecharged. On the other hand, if the non-polarity polymer molecules areadsorbed to the surface of the particles, the surface electrification ofthe particles is neutralized. Therefore, in the electrophoreticmaterial, it is possible to charge the first particles with the firstpolarity, to charge the second particles with the second polarity, andto cause the electric charge amount of second particles to be greaterthan the electric charge amount of first particles by varying a coatingratio due to the non-polarity polymer molecules in the first particlesand the second particles.

Application Example 4

In the electrophoretic material according to Application Example 1, itis preferable that the first particles have white color, and the secondparticles have color which is different from the white color.

In the electrophoretic material, since the first particles, which aredifficult to be electrophoresed, have white color, and are dispersed inthe electrophoretic material in a relatively uniform and regular manner,the scattering of light is relatively uniform and regular, and thus itis possible to implement white display with high brightness.

Application Example 5

In the electrophoretic material according to Application Example 4, itis preferable that a volume concentration of the first particles begreater than a volume concentration of the second particles.

In the electrophoretic material, since the volume ratio of the firstwhite particles is greater than the volume ratio of the second particleswhich have color and easily migrate, it is possible to implement thewhite display with high brightness.

Application Example 6

In the electrophoretic material according to Application Example 4, itis preferable that a ratio of a refractive index of the first particlesto a refractive index of the solvent be greater than 1, and the firstparticle average radius be in a range which is equal to or greater than0.05 μm and equal to or less than 0.4 μm.

In the electrophoretic material, it is possible to increase thescattering efficiency of visible light by the first particles.Therefore, it is possible to implement the white display with highbrightness.

Application Example 7

In the electrophoretic material according to Application Example 6, itis preferable that the first particle average radius be in a range whichis equal to or greater than 0.1 μm and equal to or less than 0.15 μm.

In the electrophoretic material, it is possible to particularly increasethe scattering efficiency of visible light by the first particles.Therefore, it is possible to particularly implement the white displaywith higher brightness.

Application Example 8

In the electrophoretic material according to Application Example 4, itis preferable that the first particles be resin which includes anoptical interface therein.

In the electrophoretic material, it is possible to scatter light on thesurfaces of the first particles and an internal optical interface both.Therefore, it is possible to implement the white display with highbrightness.

Application Example 9

In the electrophoretic material according to Application Example 8, itis preferable that the first particles include gaps therein.

In the electrophoretic material, it is possible to easily form theoptical interfaces in the first particles. Further, since the density ofthe first particles is less than the density of the resin and is closeto the density of the solvent, it is possible to disperse the firstparticles in the electrophoretic material in a relatively uniform andregular manner while improving dispersing stability. Therefore, it ispossible to implement the white display with high brightness and it ispossible to improve image retention characteristics.

Application Example 10

According to Application Example 10, there is provided anelectrophoresis display device including the electrophoretic material.

In the electrophoretic material, it is possible to provide theelectrophoresis display device in which a response speed is fast, inwhich clean display is performed, and in which image retentioncharacteristics are excellent.

Application Example 11

According to Application Example 10, there is provided an electronicapparatus including the electrophoretic material.

In the electrophoretic material, it is possible to provide theelectronic apparatus in which a response speed is fast, in which cleandisplay is performed, and in which image retention characteristic isexcellent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a perspective view illustrating an electronic apparatusaccording to a first embodiment.

FIGS. 2A and 2B are cross-sectional views illustrating anelectrophoresis display device according to the first embodiment.

FIG. 3 is a pattern diagram illustrating the configuration ofelectrophoretic materials.

FIGS. 4A and 4B are views illustrating the relationship between the gapand concentration of the electrophoretic materials.

FIG. 5 is a view illustrating the relationship between a particlediameter and light scattering intensity.

FIGS. 6A and 6B are views illustrating a particle charging process.

FIGS. 7A and 7B are circuit diagrams illustrating the electrophoresisdisplay device according to the first embodiment.

FIG. 8 is a perspective view illustrating the configuration ofelectronic paper.

FIG. 9 is a perspective view illustrating the configuration of anelectronic note.

FIGS. 10A and 10B are cross-sectional views illustrating anelectrophoresis display device according to a second embodiment.

FIGS. 11A and 11B are views illustrating electrophoretic materialaccording to a first modification example.

FIGS. 12A and 12B are views illustrating electrophoretic materialaccording to a second modification example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings. Also, since each layer or eachmember has been resized to a recognizable degree in each of the drawingsbelow, the scale of each layer or each member is different from anactual scale.

First Embodiment Outline of Electronic Device

FIG. 1 is a perspective view illustrating an electronic apparatusaccording to a first embodiment. First, the outline of the electronicapparatus and an electrophoresis display device according to the firstembodiment will be described with reference to FIG. 1.

As shown in FIG. 1, an electronic apparatus 100 according to theinvention includes an interface to operate an electrophoresis displaydevice 150 and the electronic apparatus 100. Specifically, the interfaceis an operation unit 120 and includes switches. The electrophoresisdisplay device 150 is a display module which includes a display area 10.The display area 10 includes a plurality of pixels 20 (refer to FIG.7A), and an image is displayed in the display area 10 in such a way thatthe pixels 20 are electrically controlled.

Structure of Electrophoresis Display Device

FIGS. 2A and 2B are cross-sectional views illustrating theelectrophoresis display device according to the first embodiment. FIG.2A illustrates first display, and FIG. 2B illustrates second display.Also, it is assumed that the color of first particles 241 is first colorand a case in which the first color is displayed is first display. Inthe same manner, it is assumed that the color of second particles 242 issecond color and a case in which the second color is displayed is seconddisplay. The first color is different from the second color. In thefirst embodiment, as an example, it is assumed that the first color is awhite color and the second color is a black color. Subsequently, thestructure of the electrophoresis display device 150 according to thefirst embodiment will be described with reference to FIGS. 2A and 2B.

The electrophoresis display device 150 includes a first substrate 80 anda second substrate 90. A plurality of pixels 20 are arranged in thedisplay area 10 in a matrix. In an area which becomes the display area10 in the first substrate 80, a pixel electrode 22 and a pixel circuit(refer to FIG. 7B) are formed for each pixel 20. In the second substrate90, a common electrode 23 is formed on the almost entire surface. Thepixel circuit includes a pixel switching transistor 21 and a holdingcapacitor 25, and a single pixel 20 is formed at the intersection of ascan line 30 and a data line 40. Further, a common electric potentialline 50 is wired in the pixel 20. An electrophoretic material 24 isinterposed between the first substrate 80 and the second substrate 90.The particles of the electrophoretic material 24 are dispersed in asolvent. The particles include at least first particles 241 which arecharged with first polarity and second particles 242 which are chargedwith second polarity, the particles are dispersed in the solvent, whichis not shown in the drawing, together with an additive agent, such as adispersing agent, which is not shown in the drawing, and configure theelectrophoretic material 24.

The first polarity and the second polarity have electrically oppositepolarities. In the first embodiment, as an example, it is assumed thatthe first polarity is negative (minus), and the second polarity ispositive (plus). As a result, the electrophoretic material 24 showselectrophoretic property between the pixel electrode 22 and the commonelectrode 23. Since the first particles 241 and the second particles 242are charged with polarities which are opposite to each other, it ispossible to control the distribution positions of the first particles241 and the second particles 242 in an electric field which is appliedto the electrophoretic material 24. That is, it is possible to stabilizethe qualities of images between different images.

It is preferable that the first color of the first particles 241 and thesecond color of the second particles 242 be complementary to each other.That is, it is preferable that a reflected color, obtained in such a waythat the second particles 242 absorb the first color when white light isemitted to the second particles 242, be the second color. In the firstembodiment, as an example, the first color is a white color, and thefirst particles 241 produce the white color. Therefore, the second colorwhich is produced by the second particles 242 is a black color.

The second substrate 90 is transparent, and a user views theelectrophoresis display device 150 from a side of the second substrate90. Therefore, as shown in FIG. 2A, for example, in order to performwhite display, the first white particles 241 are drawn on the side ofthe common electrode 23, and the second black particles 242 are drawn onthe side of the pixel electrode 22. Light which is incident upon theelectrophoresis display device 150 is scattered by the first particles241, and thus the first display is performed. In contrast, in order toperform black display, as shown in FIG. 2B, the second black particles242 are drawn on the side of the common electrode 23, and the firstwhite particles 241 are drawn on the side of the pixel electrode 22.Light which is incident upon the electrophoresis display device 150 isscattered by the second particles 242, and light having the first color(in this case, white color) is absorbed, and thus the second display (inthis case, black display) is performed.

In the first embodiment, although the second particles 242 are chargedwith the second polarity, a charging process to show the second polaritywhich is stronger than that of the first particles 241 is performed inthe second particles 242. That is, the electric charge amount of secondparticles 242 is greater than the electric charge amount of firstparticles 241. In other words, the electrophoretic mobility of thesecond particles 242 is greater than the electrophoretic mobility of thefirst particles 241. As understood when FIG. 2A is compared with FIG.2B, the second particles 242 are electrophoresed in such a way as toperceptively respond to the electric field in the first embodiment.Therefore, it is necessary that the second particles 242 are stronglycharged, and the electrophoretic mobility should be great. On the otherhand, the first particles 241 may be comparatively uniformly distributedbetween the first substrate 80 and the second substrate 90, and does notneed to be strongly electrophoresed. In conclusion, in the firstembodiment, display changes over in such a way that the second particles242 are mainly electrophoresed in the electrophoresis display device 150and the first particles 241 are weakly electrophoresed. Generally, ifthe charging process is performed on particles, ions having oppositepolarity are released in the solvent, and thus the insulation propertyof the solvent is lowered. In the electrophoretic material 24 accordingto the first embodiment, since the charging process performed on thefirst particles 241 is weak, the amount of ions which are released inthe solvent decreases, and thus the insulating property of the solventincreases. If the electric insulating property of the solvent increases,the electric field is effectively applied to the charged particles, andthus the electrophoretic property is improved. That is, since effectiveelectric field intensity which is incurred by the charged particlesincreases, the electrophoretic property is improved. In this way, theelectrophoresis display device 150 shows excellent electrophoreticproperty. In addition, the electrophoretic material 24 is configuredsuch that the second particles 242 are smaller than the first particles241, and the second particles 242 easily pass through the gaps betweenthe particles of the first particles 241. If so, the electric chargeamount becomes large, accordingly, the electrophoretic mobility islarge, and the second small particles 242 can easily pass through thegaps between the first particles 241. As a result, the response speed ofthe electrophoretic material 24 is accelerated, and thus it is possibleto drive the electrophoretic material 24 at a low voltage. In otherwords, it is possible to realize the electrophoresis display device 150having excellent electrophoretic property.

Electrophoretic Material

FIG. 3 is a pattern diagram illustrating the configuration of theelectrophoretic material. In addition, FIGS. 4A and 4B are viewsillustrating the relationship between the gap and concentration of theelectrophoretic material. FIG. 5 is a view illustrating the relationshipbetween a particle diameter and light scattering intensity. FIGS. 6A and6B are views illustrating a particle charging process. Subsequently, theelectrophoretic material 24 according to the first embodiment will bedescribed with reference to FIGS. 3 to 6B. The material 24 mainlyincludes particles, such as the first particles 241 and the secondparticles 242, a solvent, and the other additive agents. Hereinafter, ifit is not particularly necessary to distinguish the first particles 241from the second particles 242, both are collectively and simply calledparticles.

According to the research which has been thoroughly studied by theapplicant, the known reason for an electrophoretic material in therelated art having inferior electrophoretic property is that theparticle diameter or concentration of the electrophoretic particle, theelectric charge amount, and the like are left out of consideration.Here, subsequently, the optimal configuration of the electrophoreticmaterial 24 which includes the first particles 241, the second particles242, and the like will be described. First, it is assumed that theelectrophoretic material 24 includes k kinds of particles from the firstparticle 241 to the k-th particle. Here, k is an integer which is equalto or greater than 2. It is assumed that an i-th particle has a weightof W_(i) and a density of ρ_(i) in the electrophoretic material 24.Further, it is assumed that a material which combines the solvent andthe additive agent has a weight of W₀, and a density of ρ₀ in theelectrophoretic material 24. At this time, the volume V_(i) of the i-thparticle is expressed by Equation 1.

$\begin{matrix}{V_{i} = \frac{W_{i}}{\rho_{i}}} & (1)\end{matrix}$

In addition, the volume (called the entire volume of the solvent) V₀ ofthe material which combines the solvent and the additive agent isexpressed by Equation 2.

$\begin{matrix}{V_{0} = \frac{W_{0}}{\rho_{0}}} & (2)\end{matrix}$

The weight concentration C_(Wi) of the i-th particle is a ratio of theweight of the i-th particle to the entire weight of the electrophoreticmaterial 24, and is expressed by Equation 3.

$\begin{matrix}{C_{Wi} = \frac{W_{i}}{\sum\limits_{i = 0}^{k}W_{i}}} & (3)\end{matrix}$

In the same manner, the volume concentration C_(Vi) of the i-th particleis a ratio of the volume of the i-th particle to the entire volume ofthe electrophoretic material 24, and is expressed by Equation 4.

$\begin{matrix}{C_{Vi} = \frac{V_{i}}{\sum\limits_{i = 0}^{k}V_{i}}} & (4)\end{matrix}$

If the average radius of the i-th particle is called an i-th particleaverage radius and expressed as a_(i), the number concentration C_(Ni)of the i-th particle is expressed by Equation 5. As an example, theaverage radius of the first particles 241 is a first particle averageradius a₁, and the average radius of the second particles 242 is asecond particle average radius a₂. Also, the number concentration C_(Ni)is the number of i-th particles for unit volume of the electrophoreticmaterial 24.

$\begin{matrix}{C_{Ni} = {\frac{C_{Vi}}{\frac{4}{3}\pi \; a_{i}^{3}} = {\frac{1}{\frac{4}{3}\pi \; a_{i}^{3}} \cdot \frac{V_{i}}{\sum\limits_{i = 0}^{k}V_{i}}}}} & (5)\end{matrix}$

Equation 4 is taken into consideration in the second symbol in Equation5. If the number average volume of the entire k kinds of particles iscalled particle average volume and is indicated by V_(p), V_(p) is as inEquation 6.

$\begin{matrix}{V_{p} = {\frac{\sum\limits_{i = 1}^{k}{\frac{4}{3}\pi \; a_{i}^{2}C_{Ni}}}{\sum\limits_{i = 1}^{k}C_{Ni}} = {{\frac{4}{3}{\pi \cdot \frac{\sum\limits_{i = 1}^{k}{a_{i}^{3}C_{Ni}}}{\sum\limits_{i = 1}^{k}C_{Ni}}}} \equiv {\frac{4}{3}\pi \; a^{3}}}}} & (6)\end{matrix}$

Also, the rightmost symbol in Equation 6 is a definitional equationwhich relates to a, and a is called an apparent particle radius.Therefore, the apparent particle radius a is as in Equation 7.

$\begin{matrix}{a = \left( \frac{\sum\limits_{i = 1}^{k}{a_{i}^{3}C_{Ni}}}{\sum\limits_{i = 1}^{k}C_{Ni}} \right)^{\frac{1}{3}}} & (7)\end{matrix}$

In addition, the number of i-th particles N_(i) in the electrophoreticmaterial 24 is as in Equation 8.

$\begin{matrix}{N_{i} = \frac{V_{i}}{\frac{4}{3}\pi \; a_{i}^{3}}} & (8)\end{matrix}$

The volume, obtained by dividing the volume of the material whichcombines the solvent and the additive agent by the total number ofparticles in the electrophoretic material 24, is called free volume andexpressed as V_(F). The free volume V_(F) is, in short, the volume ofthe material which combines the solvent and the additive agent for asingle particle, and expressed as in Equation 9.

$\begin{matrix}{V_{F} = \frac{V_{0}}{\sum\limits_{i = 1}^{k}N_{i}}} & (9)\end{matrix}$

Subsequently, the radius of a spherical space occupied by the sum of theaverage volume V_(p) of the particles expressed in Equation 6 and thefree volume V_(F) expressed in Equation 9 is called a free volume radiusand expressed as R. From this definition, the free volume radius Rsatisfies the relationship expressed in Equation 10.

$\begin{matrix}{{\frac{4}{3}\pi \; R^{3}} \equiv {V_{p} + V_{F}}} & (10)\end{matrix}$

A ratio of the total volume of the particles to the entireelectrophoretic material 24 is called volume fraction φ. The volumefraction φ is obtained by summing up Equation 4 with regard to kparticles. Further, when the volume fraction φ is taken intoconsideration with regard to a single particle, the volume fraction φ isa ratio of the average volume V_(p) of the particles to the sum of theaverage volume V_(p) and the free volume V_(F) of the particle.Therefore, the volume fraction φ is as Equation 11.

$\begin{matrix}{\varphi = {{\sum\limits_{i = 1}^{k}C_{Vi}} = {\frac{\sum\limits_{i = 1}^{k}V_{i}}{\sum\limits_{i = 0}^{k}V_{i}} = {\frac{V_{p}}{V_{p} + V_{F}} = {\frac{\frac{4}{3}\pi \; a^{3}}{\frac{4}{3}\pi \; R^{3}} = \left( \frac{a}{R} \right)^{3}}}}}} & (11)\end{matrix}$

In the electrophoretic material 24, the first particle average radius a₁is greater than the second particle average radius a₂, and thedifference between the free volume radius R and the first particleaverage radius a₁ is greater than the second particle average radius a₂.That is, with the regard to the radius of the particle, Equation 12 issatisfied.

a ₁ >a ₂ S ₁=2R−2a ₁>2a ₂  (12)

FIG. 3 illustrates the relationship of a particle surface distance S₁(refer to as a first particle surface distance S1) between the firstparticle 241 and another first particle 241, the free volume radius R,the first particle average radius a₁, and the second particle averageradius a₂. The first particle surface distance S₁ is equal to thedifference between double free volume radius R (free volume diameter)and double first particle average radius a₁ (first particle averagediameter). On the other hand, the particle surface distance S1 isgreater than the double second particle average radius a₂ (secondparticle average diameter).

In addition, the electric charge amount of second particles 242 isgreater than the electric charge amount of first particles 241. As aresult of such configuration, the second particles 242 can easily passthrough the gaps between the first particles 241, and thus theelectrophoretic property of the electrophoretic material 24 is improved.

Further, in the electrophoretic material 24, the volume concentrationC_(V1) of the first particles 241 is greater than the volumeconcentration C_(V2) of the second particles 242. The mobility of thefirst particles 241 which have a large particle diameter and high volumeconcentration C_(V1) is smaller than that of the second particles 242.As a result, the movement rate of the first particles 241 becomes low,and thus it is possible to reduce the resistance (power) which isreceived by the solvent due to the movement of the first particles 241,the unnecessary flow of solvent is suppressed, and thus the movement ofthe second particles 242 is not inhibited. In other words, the volumeratio of the first white particles 241 is greater than the volume ratioof the second particles 242 of color which easily move, and thus whitedisplay which has high brightness is implemented. Also, the electriccharge amount Q₁ of the i-th particle in the electrophoretic material 24is defined using Equation 13.

$\begin{matrix}{\mu_{i} = {\frac{V}{E} = \frac{Q_{i}}{6\pi \; \eta \; a_{i}}}} & (13)\end{matrix}$

Where V is an average speed when an electric field E is applied to thei-th particle, η is the viscosity of the solvent, and μ_(i) is theelectrophoretic mobility of the i-th particle. Otherwise, zeta potentialζ_(i) which is defined in Equation 14 is obtained and calculated.

$\begin{matrix}{\varsigma_{i} = \frac{Q_{i}}{4\pi \; a_{i}ɛ_{0}ɛ_{S}}} & (14)\end{matrix}$

Where ∈₀ is the dielectric constant of vacuum, ∈_(S) is the relativedielectric constant of the solvent.

Whether or not Equation 12 is satisfied is examined as follows. The i-thparticle average radius a_(i) can be specified by measuring particlesize distribution. The free volume radius R is acquired based on thevolume fraction φ and the apparent particle radius a using Equation 11.The volume fraction φ is acquired by summing up the volume concentrationC_(Vi) expressed in Equation 4 with regard to the k kinds of particlesusing the first symbol in Equation 11. The apparent particle radius a isacquired by applying Equation 4 to Equation 7.

FIG. 4A is a view illustrating the relationship between the particleconcentration and the particle surface distance (refer to as a particlesurface distance S). The horizontal axis indicates the volume fractionφ, and the vertical axis is the particle surface distance S. Theparticle surface distance S is an average gap between particles, and isa value obtained by subtracting the double of the apparent particleradius a (the apparent particle diameter) from the free volume diameter2R, as shown in FIG. 4B. When Equation 11 is taken into consideration, Sis expressed as in Equation 15.

$\begin{matrix}{S = {{{2R} - {2a}} = {2a\left\{ {\left( \frac{d}{\varphi} \right)^{\frac{1}{3}} - 1} \right\}}}} & (15)\end{matrix}$

Where d is a filling ratio, a volume ratio obtained when a space isminutely filled with a sphere. The wavelength of visible light isapproximately λ=550 nm. If the particle surface distance S is less thanλ/2 (275 nm), scattering efficiency due to the particles is reduced. Ifthe particle surface distance S is greater than the λ/2 (275 nm),scattering efficiency due to the particles is improved. Therefore, it ispreferable that the volume fraction φ be equal to or less than 10%according to FIG. 4A. Since the scattering efficiency is maximized whenthe particle surface distance S is approximately λ/2 (275 nm), it ispreferable that the volume fraction φ be a value which is equal to orless than 10% and is close to 10%.

In the electrophoretic material 24, the first particles 241 have whitecolor, and the second particles 242 have color which is different fromthe white color. That is, the first particles 241, which are difficultto be electrophoresed because the particle diameters are relativelylarge and the electric charge amount is small, have white color, and aredispersed in the electrophoretic material 24 in a relatively uniform andregular manner. As a result, the scattering of light is relativelyuniform and regular, and thus it is possible to implement the whitedisplay in which the brightness is high. Above all, the volume fractionis equal to or less than 10%, and, in a case of the value of the volumefraction to be 10% after rounding off, white display which has extremelyhigh brightness is performed.

FIG. 5 is a view illustrating the relationship between a ratio ofscattering power and the particle diameter of red light R, green lightG, and blue light B. Subsequently, the particle diameter will bedescribed with reference to FIG. 5. In the electrophoretic material 24,a ratio of the refractive index of the first particles 241 to therefractive index of the solvent is greater than 1. The particles includepigment itself and particles (resin-based color particles) which areobtained by mixing a color material, called pigment or dye, withresin-based particles. When viewed from dispersing stability, it ispreferable that the particles be small. More specifically, the firstparticle average radius is in a range which is equal to or greater than0.05 μm and equal to or less than 0.4 μm. It is further preferable thatthe first particle average radius be in a range which is equal to orgreater than 0.1 μm and equal to or less than 0.3 μm. That is, it ispreferable that the average particle diameter of the first particles 241be in a range which is equal to or greater than 0.1 μm and equal to orless than 0.8 μm. It is further preferable that the average particlediameter of the first particles 241 be in a range which is equal to orgreater than 0.2 μm and equal to or less than 0.6 μm. As shown in FIG.5, in such ranges, the ratio of scattering power of the visible lighthas the maximum value. Therefore, if the first particles 241 whichperform the white display satisfy the above conditions, the scatteringefficiency becomes high, and the white brightness is improved.

When pigment is used as the particle, the pigment may be used as it is,or the surface of the pigment may be coated by a material, such as aresin material or another pigment, and may then be used. As describedabove, it is preferable that the first particles 241 be a resin whichincludes an optical interface inside thereof. The particles, which coatthe surface of the pigment using another material, include, for example,particles which coat the surfaces of titanium oxide particles withsilicon oxide or aluminum oxide. The particles are suitable as the whiteparticles. The refractive index of the silicon oxide or the aluminumoxide is close to the refractive index of the solvent, and light ishardly scattered on an interior titanium oxide surface. The siliconoxide or the aluminum oxide can suppress the photocatalytic activity ofthe titanium oxide, and can acquire white particles which have excellentweatherability. As another method of providing the optical interfacesinside the first particles 241, the first particles 241 may include gapsinside thereof. The interface between fluid (gas or liquid), which fillsup the gap, and the resin becomes the optical interface. Further, sincethe density of the fluid is mainly smaller than the density of theresin, the density of the first particles 241 is smaller than thedensity of the resin and is close to the density of the solvent. As aresult, the dispersing stability is improved, and the first particles241 are dispersed in the electrophoretic material in a relativelyuniform and regular manner. Accordingly, the electrophoresis displaydevice 150 can implement the white display which has high brightness,and can improve the image retention characteristics.

When resin-based color particles are used as the particles, as a basalmaterial of resin-based particles, acrylic resin or urethane-basedresin, urea-based resin, epoxy-based resin, polystylrene, polyester,polyethylene, polypropylene, and the like are exemplified. Theresin-based particles are obtained by combining one or two of them. Theresin-based color particles which are obtained by mixing a colormaterial with the resin-based particles include resin-based particleswhich are obtained by coating the surfaces of the resin-based particleswith pigment or dye in addition to resin-based particles which areobtained by mixing pigment or dye with the above-described basalmaterial in an appropriate composition ratio. The surfaces of theresin-based color particles obtained as described above may be furthermodified using an organic material such as macromolecular polymer, andmay be modified using an inorganic material such as silicon oxide.

Since the pigment which presents a first color is used as the firstparticles 241 and the first color is white in the first embodiment,titanium oxide (density 4 g/cm³ is s used for the first particles 241 asa case in point. The average particle diameter of the titanium oxide is250 nm. In addition to titanium oxide, antimonous oxide (density 5.2g/cm³), aluminum oxide (density 4.0 g/cm³), calcium carbonate (density2.8 g/cm³), barium oxide (density 5.7 g/cm³), barium carbonate (density4.3 g/cm³), barium sulfate (density 4.5 g/cm³), magnesium oxide (density3.65 g/cm³), and the like may be used for the first particles 241.

Since the pigment which presents a second color is used as the secondparticles 242 and the second color is black in the first embodiment,titanium black (density 4 g/cm³) is used for the second particles 242 asa case in point. The average particle diameter of the titanium black is100 nm. In addition to titanium black, aniline black, carbon black, andthe like may be used for the second particles 242.

It is preferable that a high insulating solvent be used for the solvent.As an example, water, an alcohol-based solvent, such as methanol,ethanol, butanol, isopropanol, octanol, methylcellosolve, or the like,an aromatic-based hydrocarbon system solvent, such as o-xylene orm-xylene, p-xylene, toluene, benzene, dodecylbenzene, hexyl benzene,Phenyl xylyl ethane, or the like, an alicyclic hydrocarbon systemsolvent, such as cyclohexane or naphthene-based hydrocarbon, or thelike, an aliphatic hydrocarbon-based solvent, such as n-hexane,kerosene, paraffin-based hydrocarbon, or the like, various types ofester-based solvents, such as acetic ether, butyl acetate, and the like,a ketone-based solvent, such as acetone and methyl ethyl ketone, methylisobutyl ketone, or the like, silicone oil, such as polydimethylsiloxaneor the like, plant oil, such as soy oil, sunflower oil, olive oil, orthe like, a halogenated hydrocarbon-based solvent, such as chlorobutane,chloroform, dichlorobenzene, trichlorethylene, trichlorfluoroethylene,trichloroethane, carbon tetrachloride, cyclohexylchloride,chlorobenzene, 1,1,2,2-tetrachloroethylene, fluoroethane trichloride,ethyldibromide tetrafluoride, ethane bromide, methylene iodide, triiodosilane, methyl iodide, or the like, carbon bisulfide, a petroleum-basedsolvent, such as an isoparaffin-based solvent, and the like exemplifywater and almost organic solvents. When high electrophoretic property isshown, high insulating property is obtained for a solvent. The optimalsolvent includes the silicone oil, the petroleum-based solvent, and theester-based solvent. In the ester-based solvent, ester which isdescribed in a general expression C_(n)H_(2n+1)COOC_(m)H_(2m+1) is themost preferable. In the first embodiment, ester of C₆H₁₃COOCH₃ is used.The electrophoretic material 24 is manufactured by placing whiteparticles of 30 wt % and black particles of 5 wt % into the ester-basedsolvent, applying ultrasonic waves for one hour, and dispersing them.

FIGS. 6A and 6B are views illustrating a particle charging process. FIG.6A shows the first particle, and FIG. 6B shows the second particle.Subsequently, a particle charging process method will be described withreference to FIGS. 6A and 6B. In the electrophoretic material 24, thesurface of the first particle 241 is coated with polymer molecules whichinclude a functional group having a non-polarity or a first polarity,and the surface of the second particle 242 is coated with polymermolecules which include a functional group having a second polarity.When this is done, it is possible to charge the first particle 241 withthe first polarity, and it is possible to charge the second particle 242with the second polarity. Further, since it is possible to independentlycontrol the electric charge amount of first particle 241 and theelectric charge amount of second particles 242, it is possible to causethe electric charge amount of second particles 242 to be greater thanthe electric charge amount of first particle 241.

The first particles 241 and the second particles 242 both includemetallic oxide, such as silica, alumina, zirconia, or the like, on thesurfaces thereof as necessary. Mainly, the surfaces of the particles arecharged by the disassociation of surface hydroxyl group (—OH⁻ is presenton the surfaces). A silane coupling agent or the like is used as asurface preparation agent. The silane coupling agent is an organicsilicide which combines functional polymer molecules and a hydrolyzablegroup (—OR). —OCH₃, —OC₂H₅, —OCOCH₃, and the like are used as thehydrolyzable group (—OR). After being temporarily hydrolyzed forsilanolating the silane coupling agent, the hydrolyzable group (—OR)directly dehydration-reacts to the hydroxyl group on the particlesurface, and is coupled to the particle surface.

As shown in FIG. 6A, the functional polymer molecules of the surfacepreparation agent which coats the first particle 241 are non-polaritypolymer molecules. As an example, the non-polarity polymer molecules arecarbon hydride or silicone. The surface hydroxyl group which has beenoriginally present on the surface of the first particle 241 is coupledto the surface preparation agent through covalent coupling as describedabove, and thus the surface of the first particle 241 is coated with thenon-polarity polymer molecules. As a result, the amount of surfacehydroxyl group which is negatively charged is reduced in such a way thatthe surface hydroxyl group is coupled to the surface preparation agent.That is, it is possible to control the negative electric charge amountdepending on a surface preparation agent coating amount.

As shown in FIG. 6B, the functional polymer molecules of the surfacepreparation agent which coat the second particle 242 are polymermolecules which include a functional group having the second polarity.As an example, the polymer molecules which include the functional grouphaving the second polarity are polymer molecules which include aquaternized-ammonium group or an amino group. The surface hydroxyl groupwhich has been originally present on the surface of the second particle242 is coupled to the surface preparation agent through covalentcoupling as described above and is reduced. The surface of the secondparticle 242 is coated with the polymer molecules which include thefunctional group having the second polarity. The functional group whichhas the second polarity is disassociated and is positively charged.Therefore, the electric charge amount of second particles 242 iscontrolled by increasing the amount of second polarity group in thepolymer molecules. Like this, it is possible to control the positiveelectric charge amount depending on a surface preparation agent coatingamount. The polymer molecules which include the polarity group may becopolymer or block polymer.

Also, the surface preparation agent may cause the silane coupling agentwhich includes the functional polymer molecules to be adsorbed to thesurfaces of the particles, and then may cause the silane coupling agentto react thereon, or may cause a treatment agent which includes thesilane coupling agent and a polymerization start group to be adsorbedand to react, and then the functional polymer molecules may be attachedto the silane coupling agent using a known polymerization method.

EXAMPLE

An example according to the first embodiment will be described below. Inthe electrophoretic material 24, the solvent is isoparaffin, the densityρ₀ is 0.8 g/cm³, the weight concentration C_(W0) is 65 wt %, and thevolume concentration C_(V0) is 90.3%. In the electrophoretic material24, the first particles 241 are titanium oxide (TiO₂), the density ρ₁ is4 g/cm³, the weight concentration C_(W1) is 30 wt %, and the firstparticle average radius a₁=125 nm. Therefore, the volume concentrationC_(V1) of the first particles 241 are 8.3%. In the electrophoreticmaterial 24, the second particles 242 are titanium black, the density ρ₂is 4 g/cm³, the weight concentration C_(W2) is 5 wt %, the secondparticle average radius a₂ is 50 nm. Therefore, the volume concentrationC_(V2) of the second particles 242 is 1.4%. As a result, according toEquation 11, the volume fraction φ=9.7%. On the other hand, according toEquation 5, the number concentration C_(N1) of the first particles 241is 1.02×10¹³ number/cm³, and the number concentration C_(N2) of thesecond particles 242 is 2.65×10¹³ number/cm³. When the values of thenumber concentration are substituted into Equation 7, the apparentparticle radius a is 85.8 nm. When the values of the volume fraction φand the apparent particle radius a are lastly substituted into Equation11, the free volume radius R=187 nm is acquired. 2R−2a₁=124 nm. Thisvalue is greater than 2a₂=100 nm, and satisfies Equation 12. That is,the second particles 242 can easily pass through the gap between thefirst particles 241. For reference, 2R−2a₂=274 nm. This value is greaterthan 2a₁=250 nm, and thus first particles 241 can easily pass throughthe gap between the second particles 242. As described above, in theelectrophoretic material 24 according to the first embodiment, the firstparticles 241 and the second particles 242 are mutually configured toeasily pass through the gaps between other parties, and thus anelectrophoretic response with regard to the electric field is fast.

Comparison Example

As a comparison example, a relationship of the composition disclosed inJP-A-2011-118417 is found out. In JP-A-2011-118417, the solvent isdodecyl benzene, the density ρ₀ is 0.856 g/cm³, the volume V₀ of thesolvent is 80 mL, and the weight W₀ is 68.48 g. The acrylic resin whichcorresponds to the first particles has a density ρ₁ of 1.2 g/cm³, aweight concentration C_(W1) of 9.5 wt %, and an acrylic resin averageradius a₁ of 2000 nm. Therefore, the volume concentration C_(V1) of theacrylic resin is 7.5%. The titanium oxide which corresponds to thesecond particles has a density ρ₂ of 4 g/cm³, a weight concentrationC_(W2) of 9.5 wt %, and a titanium oxide average radius a₂ of 125 nm.Therefore, the volume concentration C_(V2) of the titanium oxide is2.3%. As a result, according to Equation 11, the volume fraction φ is9.8%. On the other hand, according to Equation 5, the numberconcentration C_(N1) of the acrylic resin is 2.24×10⁹ number/cm³, andthe number concentration C_(N2) of the titanium oxide is 2.76×10¹²number/cm³. When the values of the number concentration are substitutedinto Equation 7, the apparent particle radius a is 204 nm. When thevalues of the volume fraction φ and the apparent particle radius a arelastly substituted into Equation 11, a free volume radius R of 442 nm isacquired. 2R−2a₁=−3120 nm. This value is smaller than 2a₂=250 nm, anddoes not satisfy Equation 12. That is, it is difficult that the titaniumoxide passes through the gaps between acrylic resins. On the other hand,2R−2a₂=634 nm. This value is smaller than 2a₁=4000 nm, and does notsatisfy Equation 12. That is, it is difficult that the acrylic resinpasses through the gaps between the titanium oxides. As described above,in the comparison example, two kinds of particles are mutuallyconfigured such that it is difficult for them to pass through the gapsbetween other parties, and thus it is considered that an electrophoreticresponse with regard to the electric field is slow.

Circuit Configuration

FIGS. 7A and 7B are circuit diagrams illustrating the electrophoresisdisplay device according to the first embodiment. FIG. 7A is a viewillustrating a whole configuration, and FIG. 7B is a view illustrating apixel circuit. Subsequently, the driving circuit of the electrophoresisdisplay device according to the first embodiment will be described withreference to FIGS. 7A and 7B.

As shown in FIG. 7A, in the display area 10, the pixels 20 correspondingto m row×n column are arranged in a matrix (two-dimensional plane). Inaddition, in the display area 10, m scan lines 30 (that is, scan linesY1, Y2, . . . , Ym) and n data lines 40 (that is, data lines X1, X2, . .. , Xn) are provided to intersect each other. Specifically, the m scanlines 30 are extended in the row direction (that is, X direction), andthe n data lines 40 are extended in the column direction (that is, Ydirection). The pixels 20 are arranged to correspond to theintersections between the m scan lines 30 and the n data lines 40.

The driving circuit 70 is provided on the outside of the display area10. The driving circuit 70 includes the controller 71, the scan linedriving circuit 72, the data line driving circuit 73, and the commonelectric potential supply circuit 74. The controller 71 controls theoperations of the scan line driving circuit 72, the data line drivingcircuit 73, and the common electric potential supply circuit 74, andsupplies various signals, such as a clock signal and a timing signal, toeach circuit.

The scan line driving circuit 72 sequentially supplies a scan signal toeach of the scan lines Y1, Y2, . . . , Ym in a pulse-like manner basedon the timing signal supplied from the controller 71. The data linedriving circuit 73 supplies an image signal to each of the data linesX1, X2, . . . , Xn based on the timing signal supplied from thecontroller 71. The image signal takes multi-valued potential from amonghigh potential VH (for example, 15 V), middle potential VM (for example,0V), and low potential VL (for example, −15 V). Also, in the firstembodiment, the image signal has three values, an image signal havinghigh potential VH is supplied to a pixel electrode 22 which istransferred to the second display (black display), and an image signalhaving low potential VL is supplied to the pixel electrode 22 which istransferred to the first display (white display).

The common electric potential supply circuit 74 supplies commonpotential Vcom to the common electric potential line 50. Also, thecommon potential Vcom may be a constant potential, and may varydepending on, for example, a grayscale or a frame to be written. Inaddition, individual control may be performed for each row. In the firstembodiment, the common potential Vcom functions as a reference potentialwith regard to the total potential. Also, although various signals areinput to and output from the controller 71, the scan line drivingcircuit 72, the data line driving circuit 73, and the common electricpotential supply circuit 74, signals which are not particularly relatedto the first embodiment will not be described.

As shown in the circuit diagram of FIG. 7B, the pixel 20 includes apixel switching transistor 21, a pixel electrode 22, a common electrode23, an electrophoretic material 24, and a holding capacitor 25.

The pixel switching transistor 21 is configured from, for example, anN-type transistor. Here, although an upper gate-type thin filmtransistor is used as shown in FIG. 2, a lower gate-type thin filmtransistor may be used. The pixel switching transistor 21 has a gatewhich is electrically connected to the scan line 30, a source which iselectrically connected to the data line 40, and a drain which iselectrically connected to each one end of the pixel electrode 22 and theholding capacitor 25. The holding capacitor 25 includes a pair ofelectrodes which are arranged to face each other via a dielectric film.One electrode (one end) is electrically connected to the pixel electrode22 and the pixel switching transistor 21, and the other electrode (theother end) is electrically connected to the common electric potentialline 50. The holding capacitor 25 enables an image signal to bemaintained for a predetermined period. The pixel switching transistor 21outputs the image signal, which is supplied from the data line drivingcircuit 73 via the data line 40, to the pixel electrode 22 and theholding capacitor 25 at a timing based on the scan signal which issupplied from the scan line driving circuit 72 via the scan line 30 in apulse-like manner.

The image signal is supplied to the pixel electrode 22 from the dataline driving circuit 73 via the data line 40 and the pixel switchingtransistor 21. The pixel electrode 22 is arranged to mutually face thecommon electrode 23 via the electrophoretic material 24. The commonelectrode 23 is electrically connected to the common electric potentialline 50 which provides the common potential V_(com). The commonelectrode 23 is provided on the second substrate 90 which faces thefirst substrate 80 on which the pixel electrode 22 is formed. The firstparticles 241 and the second particles 242 are electrophoresed in thevertical direction in the cross-sectional views shown in FIGS. 2A and2B. Also, the common electrode 23 may be provided on the first substrate80 on which the pixel electrode 22 is formed, and the electrophoreticparticles may be electrophoresed in the lateral direction (in thehorizontal direction in FIGS. 2A and 2B) of the cross-sectional views inFIGS. 2A and 2B.

Electronic Device

Subsequently, an electronic apparatus to which the above-describedelectrophoresis display device is applied will be described withreference to FIGS. 8 and 9. Hereinafter, a case in which theabove-described electrophoresis display device 150 is applied toelectronic paper 400 and an electronic note 500 is exemplified.

FIG. 8 is a perspective view illustrating the configuration of theelectronic paper. As shown in FIG. 8, the electronic paper 400 includesthe electrophoresis display device 150 according to the firstembodiment. The electronic paper 400 includes a rewritable sheet whichis flexible and has the same texture and flexibility as paper in therelated art.

FIG. 9 is a perspective view illustrating the configuration of theelectronic note. As shown in FIG. 9, the electronic note 500 is obtainedby bundling a plurality of pieces of electronic paper 400 shown in FIG.8 and sandwiching the electronic paper 400 in a cover 501. The cover 501includes, for example, a display data input unit to input display datawhich is transmitted from an external device. Therefore, it is possibleto modify or update display content based on the display data in a statein which the electronic paper 400 is bundled.

Since the above-described electronic paper 400 and the electronic note500 include the electrophoresis display device 150 according to thefirst embodiment, it is possible to perform high-quality image display.Also, it is possible to apply the electrophoresis display device 150according to the first embodiment to an electronic apparatus, such as awristwatch, a mobile phone, a portable audio device, an electronictablet terminal, a personal computer, a digital signage, an electronicblackboard, or the like, in addition to the electronic paper 400 and theelectronic note 500.

As described above, according to the electrophoretic material 24according to the first embodiment, the electrophoresis display device150 to which the electrophoretic material 24 is applied, and theelectronic apparatus to which the electrophoresis display device 150 isapplied, the following advantages can be acquired.

Since the electric charge amount of the second particles 242 each havinga small particle diameter is greater than the electric charge amount ofthe first particles 241 each having a large particle diameter, theelectrophoretic mobility of the second particles 242 is greater than theelectrophoretic mobility of the first particles 241, and thus it is easyto perform electrophoresis. On the other hand, since the second particleaverage radius a₂ is smaller than a distance obtained by subtracting thefirst particle average radius a₁ from the free volume radius R, thesecond particles 242 can easily pass through the gaps between the firstparticles 241. In short, the second particles 242 having largeelectrophoretic mobility can easily pass through the gaps between thefirst particles 241. Therefore, it is possible to drive theelectrophoretic material 24 at a low voltage by increasing the responsespeed of the electrophoretic material 24. In other words, it is possibleto provide the electrophoretic material 24 having excellentelectrophoretic property. Accordingly, it is possible to provide theelectrophoresis display device 150 and the electronic apparatus in whicha response speed is fast, in which clean display is performed, and inwhich the image retention characteristics are excellent.

Second Embodiment Forms of Three Particle Groups

FIGS. 10A and 10B are cross-sectional views illustrating anelectrophoresis display device according to a second embodiment. FIG.10A illustrates second display, and FIG. 10B illustrates third display.Hereinafter, an electrophoretic material 24 and an electrophoresisdisplay device 150 according to the second embodiment will be described.Also, the same reference numerals are used for the same components as inthe first embodiment, and the description will not be repeated.

The second embodiment (FIGS. 10A and 10B) is different from the firstembodiment (FIGS. 2A and 2B) in that there are three particle groups.The other configurations are almost the same as in the first embodiment.In the first embodiment (FIGS. 2A and 2B), there are two kinds ofparticles, that is, the first particles 241 and the second particles242. However, in the second embodiment, there are three kinds ofparticles, that is, the first particles 241, the second particles 242,and third particles 243, and thus k=3. In a case of the three particlegroups, a first particle average radius a₁ is greater than a secondparticle average radius a₂, the second particle average radius a₂ isequal to or greater than a third particle average radius a₃, thedifference between a free volume radius R and the first particle averageradius a₁ is greater than the second particle average radius a₂, and thedifference between the free volume radius R and the second particleaverage radius a₂ is greater than the third particle average radius a₃.Therefore, obviously, the first particle average radius a₁ is greaterthan the third particle average radius a₃, and the difference betweenthe free volume radius R and the first particle average radius a₁ isgreater than the third particle average radius a₃. That is, with regardto the radius of a particle, Equation 16 is satisfied.

a ₁ >a ₂ ≧a ₃ S ₁=2R−2a ₁>2a ₂ S ₂=2R−2a ₂>2a ₃  (16)

When this is done, since the second particles 242 and the thirdparticles 243 are smaller than the gaps between the first particles 241,the second particles 242 and the third particles 243 easily pass throughthe gaps between the first particles 241, and thus the response of theelectrophoretic material 24 becomes fast. At the same time, the firstparticles 241 are smaller than the gaps between the second particles 242and the gaps between the third particles 243. In addition, since thethird particles 243 are smaller than the gaps between the secondparticles 242, the third particles 243 easily pass through the gapsbetween the second particles 242, and thus the response of theelectrophoretic material 24 becomes fast. At the same time, since thesecond particles 242 are smaller than the gaps between the thirdparticles 243, the response is fast.

Equation 17 is acquired based on Equation 16.

2R−a ₁ −a ₂>2a ₃ 2R−a ₁ −a ₃>2a ₂  (17)

As understood from Equation 17, since the third particles 243 aresmaller than the gaps between the first particles 241 and the secondparticles 242, electrophoretic response is fast. Further, since thesecond particles 242 are smaller than the gaps between the firstparticles 241 and the third particles 243, the electrophoretic responseis fast.

With regard to the electric charge amount of particles, the electriccharge amount Q₁ of the first particles 241 is less than the electriccharge amount Q₂ of the second particles 242, and the electric chargeamount Q₂ of the second particles 242 is equal to or less than theelectric charge amount Q₃ of the third particles 243. As a result, themobility μ₁ of the first particles 241 is smaller than the mobility μ₂of the second particles 242, and the mobility μ₂ of the second particles242 is equal to or smaller than the mobility μ₃ of the third particles243.

With regard to three kinds of particle color, although randomcombination is possible, it is preferable that the three of white color,black color, red color, green color, blue color, cyan, magenta, yellow,and the like be assigned to a first color, a second color, and a thirdcolor. As described above, in the second embodiment, since it is mostdifficult to electrophorese the first particles 241, it is preferablethat the first color be selected as white color, the third color of thethird particles 243, which are most easy to electrophorese, be selectedas black color, and the second color is selected from among red color,green color, blue color, cyan, magenta, yellow, and the like. Also, whenthe first display is performed, two pixel electrodes are prepared foreach pixel, the second particles 242 are accumulated in one pixelelectrode, and the third particles 243 are accumulated in the otherpixel electrode.

As described above, according to the electrophoretic material 24 and theelectrophoresis display device 150 according to the second embodiment,it is possible to acquire the following advantages in addition to theadvantages of the first embodiment. Since the composition is structuredsuch that three kinds of particles do not mutually disturb theelectrophoresis, it is possible to implement high-speed response withregard to the variation in electric fields in the electrophoresisdisplay device using the three kinds of particles.

Also, the invention is not limited to the above-described embodiments,and it is possible to add various modifications and improvement to theabove-described embodiments. Modification examples will be describedbelow.

First Modification Example First Form Having Different Electric-ProofProcess

FIGS. 11A 11B are views illustrating an electrophoretic materialaccording to a first modification example. FIG. 11A illustrates a firstparticle, and FIG. 11B illustrates a second particle. Hereinafter, anelectrophoretic material 24 according to the first modification examplewill be described. Also, the same reference numerals are used for thesame components as in the first embodiment, and the description will notbe repeated.

The first modification example (FIGS. 11A and 11B) is different from thefirst embodiment (FIGS. 6A and 6B) in that a different electric-proofprocess is performed on a particle. The other configurations are almostthe same as in the first embodiment. In the first embodiment (FIGS. 6Aand 6B), the surfaces of the first particles 241 are coated with thepolymer molecules having the non-polarity (non-polarity polymermolecules), the surfaces of the second particles 242 are coated with thepolymer molecules which include the functional group having the secondpolarity. In contrast, in the first modification example, the surfacesof the first particles 241 are coated with the polymer molecules whichinclude the functional group having the first polarity. With this, thefirst particles 241 may be charged with the first polarity.

As shown in FIG. 11A, the functional polymer molecules of the surfacepreparation agent which coats the first particle 241 are the polymermolecules which include the functional group having the first polarity.As an example, the polymer molecules which include the functional grouphaving the first polarity are the polymer molecules which include acarboxyl group. Although the surface hydroxyl group which is originallypresent on the surface of the first particle 241 is reduced through thecovalent coupling with the silane coupling agent, the surface of thefirst particle 241 is coated with the polymer molecules which includethe functional group having the first polarity. The functional grouphaving the first polarity is disassociated and is negatively charged. Inthis way, the electric charge amount of the first particles 241 iscontrolled.

As shown in FIG. 11B, although the functional polymer molecules of thesurface preparation agent which coats the second particle 242 are thepolymer molecules which include the functional group having the secondpolarity, control is made such that the electric charge amount of thesecond particles 242 is greater than the electric charge amount of thefirst particles 241 by increasing the amount of the second polaritygroups in the polymer molecules. In this way, it is possible to controlthe positive electric charge amount based on the amount of coatedsurface preparation agent.

Second Modification Example Second Form Having Different Electric-ProofProcess

FIGS. 12A and 12B are views illustrating an electrophoretic materialaccording to a second modification example. FIG. 12A illustrates a firstparticle, and FIG. 12B illustrates a second particle. Hereinafter, anelectrophoretic material 24 according to the second modification examplewill be described. Also, the same reference numerals are used for thesame components as in the first embodiment, and the description will notbe repeated.

The second modification example (FIGS. 12A and 12B) is different fromthe first embodiment (FIGS. 6A and 6B) in that a differentelectric-proof process is performed on a particle. The otherconfigurations are almost the same as in the first embodiment. In thefirst embodiment (FIGS. 6A and 6B), the surface of the first particle241 is coated with the polymer molecules having the non-polarity(non-polarity polymer molecules), the surface of the second particle 242is coated with the polymer molecules which include the functional grouphaving the second polarity. In contrast, in the second modificationexample, the surface of the first particle 241 and the surface of thesecond particle 242 both are coated with the non-polarity polymermolecules. However, a coating ratio of the surface of the first particle241 is different from a coating ratio of the surface of the secondparticle 242. In this way, the charged polarities and the electriccharge amount of the first particle 241 and the second particle 242 maybe controlled.

As shown in FIGS. 12A and 12B, the surfaces of the first particle 241and the second particle 242 are coated with the non-polarity polymermolecules, the coating rate of the surface of the first particle 241 bythe non-polarity polymer molecules is greater than the coating rate ofthe surface of the second particle 242 by the non-polarity polymermolecules. If particles are dispersed in the solvent, there occurs anelectric charge separation phenomenon, which moderates work functiondifference between the particles and the solvent, and thus the particlesare charged. In FIGS. 12A and 12B, the first particle 241 and the secondparticle 242 both are positively charged. At the same time, theparticles are negatively charged due to the disassociation of thesurface hydroxyl group. On the other hand, if the non-polarity polymermolecules are adsorbed to the hydroxyl group of the surface of theparticles and reacts thereon, the negative charge of the particles dueto the hydroxyl group is neutralized. Therefore, it is possible tocharge the first particles 241 with the first polarity, to charge thesecond particles 242 with the second polarity, and to cause the electriccharge amount of the second particles 242 to be greater than theelectric charge amount of the first particles 241 by varying the coatingrate due to the non-polarity polymer molecules in the first particles241 and the second particles 242.

This application claims the benefit of Japanese Patent Application No.2012-157224, filed on Jul. 13, 2012, which is hereby incorporated byreference as if fully set forth herein.

What is claimed is:
 1. An electrophoretic material, wherein: particlesare dispersed in a solvent, volume, which is obtained by dividing avolume of the solvent by a total number of particles, is called freevolume, and a radius of a spherical space, which is occupied by sum ofan average volume of the particles and the free volume, is called a freevolume radius; the particles include at least first particles which arecharged with a first polarity and second particles which are chargedwith a second polarity, an average radius of the first particles iscalled a first particle average radius, and an average radius of thesecond particles is called a second particle average radius; the firstparticle average radius is greater than the second particle averageradius; and a difference between the free volume radius and the firstparticle average radius is greater than the second particle averageradius.
 2. The electrophoretic material according to claim 1, whereinsurfaces of the second particles are coated with polymer molecules whichinclude a functional group having the second polarity.
 3. Theelectrophoretic material according to claim 1, wherein surfaces of thefirst particles and the second particles are coated with non-polaritypolymer molecules, and wherein a coat ratio due to the non-polaritypolymer molecules on the surfaces of the first particles is greater thana coat ratio due to the non-polarity polymer molecules on the surfacesof the second particles.
 4. The electrophoretic material according toclaim 1, wherein the first particles have white color, and the secondparticles have color which is different from the white color.
 5. Theelectrophoretic material according to claim 4, wherein a volumeconcentration of the first particles is greater than a volumeconcentration of the second particles.
 6. The electrophoretic materialaccording to claim 4, wherein a ratio of a refractive index of the firstparticles to a refractive index of the solvent is greater than 1, andwherein the first particle average radius is in a range which is equalto or greater than 0.05 μm and equal to or less than 0.4 μm.
 7. Theelectrophoretic material according to claim 6, wherein the firstparticle average radius is in a range which is equal to or greater than0.1 μm and equal to or less than 0.15 μm.
 8. The electrophoreticmaterial according to claim 4, wherein the first particles are resinwhich includes an optical interface therein.
 9. The electrophoreticmaterial according to claim 8, wherein the first particles include gapstherein.
 10. The electrophoretic material according to claim 1, whereinthe particles include a third particles, an average radius of the thirdparticles is called a third particle average radius, the second particleaverage radius is equal to or greater than the third particle averageradius, and a difference between the free volume radius and the secondparticle average radius is greater than the third particle averageradius.
 11. The electrophoretic material according to claim 10, whereinthe electric charge amount of the second particles is equal to or lessthan an electric charge amount of the third particles.
 12. Theelectrophoretic material according to claim 11, wherein an electriccharge amount of the second particles is greater than an electric chargeamount of the first particles.
 13. An electrophoresis display devicecomprising: the electrophoretic material according to claim
 1. 14. Anelectronic apparatus comprising: the electrophoresis display deviceaccording to claim 13.